LHC

b-PU

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14-0

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/02/

2014

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LHCb-PUB-2014-018

Comparative Measurements of the Photon Detection Efficiency of KETEK

SiPM Detectors for the LHCb SciFi Upgrade Project

Evgeny Gushchin, Christian Joram

23 May 2013

2

Introduction The LHCb SciFi detector is conceived to employ arrays of SiPM detectors to detect scintillation light

from ribbons of 2.5 m long scintillating fibres of 250 m diameter. The fibres of type Kuraray SCSF-78

are blue emitting with an emission maximum at 440 nm. However, as a consequence of the radiation

damage mainly from charged hadrons in the LHCb experiments, the effective emission spectrum at

the end of the fibre will shift to longer wavelengths. A simulation of the light absorption in the fibre,

assuming an ionizing dose distribution along the fibre as predicted by the FLUKA code, is able to

predict the emission spectrum. Fig. 1 shows the emission spectra (in arbitrary units) for 10 cm

intervals along the fibre. At 250 cm, where the ionization dose is expected to reach over the full

lifetime of the upgrade LHCb detector about 30 kGy, the average wavelength of emission is

approximately 500 nm. The sensitivity spectrum of the SiPM detector should be tuned to match this

emission spectrum, i.e. the PDE should be as high as possible in the range from 400 to 600 nm with a

maximum around 500 nm.

Figure 1: Simulated effective emission spectra, as measurable at the end of the fibre (the readout end corresponds to 0 cm).

3

This working document summarizes preliminary results of comparative measurements of the Photon

Detection Efficiency (PDE) of KETEK SiPM detectors, produced in 2012 for the LHCb SciFi upgrade

project (LHCb 1st prototyping). The aim of these measurements is to provide feedback to the supplier

in order to tune the sensitivity spectrum for the LHCb second prototyping, which is foreseen for the

second half of 2013.

Method Our method consists of a two-step process. In the set-up, shown in Fig. 2, the SiPM detector under

test (DUT) is exposed to monochromatic light, produced in a (double-)monochromator fed with

quasi-white light from a Xenon light source. The monochromatic light at the output slit is attenuated

by a neutral density filter ND1.5 (attenuation factor 101.5 ≈ 30) and then guided through a clear

plastic fibre of 1 mm diameter to the DUT, or alternatively, to an absolutely calibrated PIN diode

(Newport 818-UV, 10 mm diameter, rel. uncertainty of QE = 2%). While the PIN diode detects all

light emitted from the fibre, the DUT is placed at a measured distance from the fibre end (around 65

mm), where the beam spot has a size of about 2 cm in diameter. It was verified that its intensity is

uniform within about 5 mm around the centre, i.e. imperfections in the positioning of the DTUT have

no impact. Depending on the detector/channel size, only a relatively small fraction of the emitted

light is detected. The relative sensitivity of the DUT is determined by measuring the photocurrent

with a Picoammeter at a given wavelength and relating it to the photocurrent of the PIN diode.

Srel,DUT = QEPD · IDUT / IPD

The dark current of the DUT, measured while the monochromator was set to a wavelength of 200

nm, was subtracted from the photocurrent of the DUT.

Figure 2

Xe lamp

“wh

(double) monochro

Clear fibre

(1

Newport 818-UV photodiode

with absolute sensitivity calibration (NIST)

1

ND filter x 1/30

DUT

4

The spectrum of the Xenon lamp is a superposition of many Doppler broadened atomic lines, which

leads to a quasi-white colour. As shown in the Figure 3, there are however several visible lines

particularly between 450 and 500 nm. Slight differences in the wavelength selected for the DUT and

the calibration run can lead to small spikes in the sensitivity data.

Figure 3: Illustration of the typical emission spectrum of a Xenon gas discharge lamp.

The photocurrent was measured in 5 nm intervals from 300 to 700 nm. The points below 310 nm

suffer from fluctuations due to very low light levels available in the UV.

The method relies on the stability of the light intensity during the two measurements are performed.

While the light intensity was found to be stable (<1%) over time scales of minutes, long term drifts

(hours) of the order 5% were observed. We therefore monitor the intensity of the white light with a

second PIN diode and apply corrections to the sensitivity data.

In a second step, in order to allow for comparison of different detectors, a number of corrections

need to be determined and applied to the sensitivity value:

Geometrical size: The rel. sensitivity of detectors is scaled according to the geometrical area

of a channel or a full detector.

The current measurement includes contributions from avalanche gain G, inter-pixel cross-

talk XT and after-pulses AP.

Srel,DUT, corr = Srel,DUT · ADUT / Aref / G / (1+XT) / (1+AP)

5

G, XT and AP were measured separately in another set-up1 as a function of the overvoltage (OV) and

corrected for. The measurement is based on a detailed waveform analysis of the SiPM pulses at =

470 nm. This second set-up also allows to determine the relative PDE (at 470 nm) from the Poisson-

zero method.

All measurements were performed at a temperature of 25±1 °C. The impact of these deviations

(±1K) on the overvoltage was ignored.

For the purpose of this comparison, the value of Srel,DUT, corr, measured for the 50D ADV detector at

OV = 4V serves as reference and was set to 1. All results have to be considered in relation to this

value.

List of measured detectors and measurements The list of the detectors tested in the measurement campaign is shown in the table 1.

The “50D-adv“ was taken as a reference to the 2011 year generation of KETEK SiPM. Similar to the

MPPC-50 mu detector from HPK it has a 1 mm2 area and 50x50 µm pixel size.

The detectors with type name starting with “W” are from 2012 batch. All of them are the arrays of

32 channels with channel size 1.32x0.25 mm.

For the measurements we bias only individual channels of the detectors.

W1,W2 – are the detectors with “standard” structure close to “50-D adv” type, but the pixel

size is 60x57.5 µm.

W5 and W6 types have additional epitaxial layer of 0.8 µm depth. Because of this fact, the

capacitance and gain of W5 and W6 pixels are smaller by ~1.5-1.6 times if compared with

“standard” structure.

Versions “2A”,”1B”,”3B”… code for the type of trench design and value of quenching

resistor: 1) “1A” or “1B” are versions without trenches, “2A” and “2B” have trenches

between channels and version “3B” has trenches between pixels ; 2)”A” type has lager

resistor than “B” type. The last number (“1” or “2”) in type name shows the serial number of

chip. All the new chips (“W”) are not covered with epoxy except the “W1-1A-2” chip which

was covered at CERN with epoxy and a thin glass window on top.

All detectors were measured at temperatures around 25 deg.C. The light intensity was kept low

enough (IDUT < 20 A) such that saturation of pixels was safely avoided. The KETEK detectors were

measured at OV = 1.5, 2.5, 3.5 and 4V. The HPK MPPC was measured at OV = 0.9, 1.1, 1.3 and 1.5V.

1 E. Gushchin, to be published.

6

Table 1: List of detectors included in the measurement campaign

Detector Comment

50D ADV KETEK standard SiPM, epoxy covered. 1 mm2

W6-1B-1 1 channel = 0.33 mm2, additional epitaxial layer, low quenching resistor

W2-1B-2 1 channel = 0.33 mm2, low quenching resistor

W6-2A-1 1 channel = 0.33 mm2, additional epitaxial layer, trenches between channels

W5-1B-1 1 channel = 0.33 mm2, additional epitaxial layer, low quenching resistor

W1-1A-2 1 channel = 0.33 mm2, epoxy covered

W1-3B-1 1 channel = 0.33 mm2, trenches between pixels, low quenching resistor

HPK MPPC-50 mu Hamamatsu standard MPPC, epoxy covered. 1 mm2.

In table 2 the results of measurements of the pixel gain and cross-talk values for various over-voltage

values are shown. The results were obtained by analysis of DUT pulses recorded by a digital

oscilloscope. To speed up the measurements, the DUT was illuminated by constant light flux from

LED. The pixel gain was obtained from measurement of current through DUT and counting the

frequency of pulses and their amplitudes. The cross-talk values are extracted from ratio of pulse

frequencies as function of amplitude. In order to prevent the overlapping of pulses a fast shaper

(~6nsec) was used.

Table 2: Results of Gain, Cross-talk and After Pulse measurements

Detector G (106) XT AP

OV 1.5 V 2.5 V 3.5 V 4 V 1.5 V 2.5 V 3.5 V 4 V

50D ADV 2.323 3.929 5.529 6.443 0.036 0.093 0.16 0.195 negl.

W6-1B-1 1.985 3.441 4.724 5.399 0.054 0.139 0.248 0.302 negl.

W2-1B-2 2.960 5.197 7.121 8.139 0.032 0.085 0.152 0.181 negl.

W6-2A-1 2.189 3.492 5.031 5.750 0.066 0.165 0.255 0.305 negl.

W5-1B-1 2.182 3.505 4.883 5.581 0.056 0.133 0.220 0.263 negl.

W1-1A-2 3.368 5.467 7.777 8.887 0.040 0.096 0.167 0.200 negl.

W1-3B-1 2.964 4.916 6.872 7.771 0.018 0.038 0.073 0.090 negl.

OV 0.9 V 1.1 V 1.3 V 1.5 V 0.9 V 1.1 V 1.3 V 1.5 V 0.9-1.5V

MPPC-50 mu 0.610 0.747 0.880 1.025 0.050 0.065 0.091 0.113

3-14%

The contribution of after pulses for KETEK detectors is small and was neglected in this study. For the

HPK MPPC-50 mu, afterpulse contributions ranging from 3 to 14% (for the considered OV values0

were found.

Results In the following we present the preliminary results of the analysis in form of rel. sensitivity plots,

relative to the 50D ADV detector at OV = 4V, for which the peak sensitivity (at 440 nm) was

normalized to 1.

All plots show on the x-axis the wavelength in nm and on the y-axis the relative PDE.

7

Comment: data for OV = 1.5 V was measured but not recorded. To be repeated.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

50D ADV KETEK 50D ADV OV=1.5V

KETEK 50D ADV OV=2.5V

KETEK 50D ADV OV=3.5V

KETEK 50D ADV OV=4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

KETEK 50D adv, normalized to 1., varying OV KETEK 50D ADV OV=1.5V

KETEK 50D ADV OV=2.5V

KETEK 50D ADV OV=3.5V

KETEK 50D ADV OV=4V

8

0.000

0.200

0.400

0.600

0.800

1.000

1.200

200 300 400 500 600 700 800

Titl

e

Title

W6-1B-1 W6-1B-1 OV = 1.5V

W6-1B-1 OV = 2.5V

W6-1B-1 OV = 3.5V

W6-1B-1 OV = 4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W6-2A-1, normalized to 1, varying OV W6-2A-1 OV=1.5V

W6-2A-1 OV=2.5V

W6-2A-1 OV=3.5V

W6-2A-1 OV=4V

9

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

200 300 400 500 600 700 800

Titl

e

Title

W2-1B-2 W2-1B-2 OV = 1.5V

W2-1B-2 OV = 2.5V

W2-1B-2 OV = 3.5V

W2-1B-2 OV = 4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W2-1B-2, normalized to 1, varying OV W2-1B-2 OV=1.5V

W2-1B-2 OV=2.5V

W2-1B-2 OV=3.5V

W2-1B-2 OV=4V

10

0.000

0.200

0.400

0.600

0.800

1.000

1.200

200 300 400 500 600 700 800

Titl

e

Title

W6-2A-1 W6-2A-1 OV = 1.5V

W6-2A-1 OV = 2.5V

W6-2A-1 OV = 3.5V

W6-2A-1 OV = 4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W6-2A-1, normalized to 1, varying OV W6-2A-1 OV=1.5V

W6-2A-1 OV=2.5V

W6-2A-1 OV=3.5V

W6-2A-1 OV=4V

11

0.000

0.200

0.400

0.600

0.800

1.000

1.200

200 300 400 500 600 700 800

Titl

e

Title

W5-1B-1 W5-1B-1 OV = 1.5V

W5-1B-1 OV = 2.5V

W5-1B-1 OV = 3.5V

W5-1B-1 OV = 4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W5-1B-1, normalized to 1, varying OV W5-1B-1 OV=1.5V

W5-1B-1 OV=2.5V

W5-1B-1 OV=3.5V

W5-1B-1 OV=4V

12

0.000

0.200

0.400

0.600

0.800

1.000

1.200

200 300 400 500 600 700 800

Titl

e

Title

W1-1A-2 W1-1A-2 OV = 4V

W1-1A-2 OV = 2.5V

W1-1A-2 OV = 3.5V

W1-1A-2 OV = 1.5V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W1-1A-2, normalized to 1, varying OV W1-1A-2 OV=1.5V

W1-1A-2 OV=2.5V

W1-1A-2 OV=3.5V

W1-1A-2 OV=4V

13

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

200 300 400 500 600 700 800

Titl

e

Title

W1-3B-1 W1-3B-1 OV = 1.5V

W1-3B-1 OV = 2.5V

W1-3B-1 OV = 3.5V

W1-3B-1 OV = 4V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

W1-3B-1, all normalized to 1., varying OV W1-3B-1 OV=1.5V

W1-3B-1 OV=2.5V

W1-3B-1 OV=3.5V

W1-3B-1 OV=4V

14

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

200 300 400 500 600 700 800

Titl

e

Title

MPPC-50mu MPPC-50mu OV=0.9V

MPPC-50mu OV=1.1V

MPPC-50mu OV=1.3V

MPPC-50mu OV=1.5V

0.00

0.20

0.40

0.60

0.80

1.00

1.20

200 300 400 500 600 700 800

Titl

e

Title

MPPC-50mu, all normalized to 1., varying OV

MPPC-50mu OV=0.9V

MPPC-50mu OV=1.1V

MPPC-50mu OV=1.3V

MPPC-50mu OV=1.5V

15

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

200 300 400 500 600 700 800

Titl

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Title

KETEK comparison (surface corrected to 1mm2), OV=4V

KETEK 50D ADV OV=4VW2-1B-2 OV=4VW6-2A-1 OV=4VW6-1B-1 OV=4VW5-1B-1 OV=4VW1-1A-2 OV=4V epoxy+glassW1-3B-1 OV=4V trenches

0

0.2

0.4

0.6

0.8

1

1.2

200 300 400 500 600 700 800

Titl

e

Title

KETEK comparison, all normalized to 1., OV=4V

W2-1B-2 OV=4VW6-2A-1 OV=4VW6-1B-1 OV=4VKETEK 50D ADV OV=4VW5-1B-1 OV=4VW1-1A-2 OV=4VW1-3B-1 OV=4V

16

Comparison of monochromator and zero pulse method An independent method of determining the relative PDE of a SiPM detector consists of exposing it to

low light levels from a pulsed LED. Assuming Poisson statistics, the measurement of the probability

to detect zero photons P(0,) = e- is related to the average number of detected photons . The

method has the advantage that crosstalk and afterpulses do not play a role in the zero

measurement. In our case, a LED with a wavelength of 470 nm was used.

To allow comparison with the data from the monochromator set-up, the pulse measurements of the

50D ADV detector was scaled to coincide at OV = 4V.

The following plot shows the rel. PDE at 470 nm, obtained with the zero pulse and the

monochromator methods method, versus the overvoltage.

0

0.2

0.4

0.6

0.8

1

1.2

0.00 1.00 2.00 3.00 4.00 5.00

Re

l. P

DE

OV (V)

Relative PDE (470 nm), normalized to 1mm2, pulse method

W2-1B-2

W6-2A-1

W6-1B-1

50D-adv

W5-1B-1

W1-1A-2(epoxy)

W1-3B-1

MPPC 1mm

17

0.000

0.200

0.400

0.600

0.800

1.000

1.200

0.00 1.00 2.00 3.00 4.00 5.00

Re

l. P

DE

OV (V)

Relative PDE (470 nm), normalized to 1mm2, monochromator

W2-1B-2

W6-2A-1

W6-1B-1

50D-adv

W5-1B-1

W1-1A-2(epoxy)W1-3B-1

MPPC 1mm

MPPC, APcorr

18

The following plot shows the correlation between the two measurements. The Iine connects the 4

measurements performed at the different overvoltages. In case of perfect agreement, all lines would

be on the diagonal. The fact that most of the lines are below the diagonal indicates that the pulse

method gives generally higher results than the monochromator set-up. Which of the two methods is

the correct one is unclear at this stage and requires a more detailed analysis.

Quantitatively, the following plot visualized the relative differences (in percent) of the two methods.

Typically, the agreement is within 10% and seems to improve for higher overvoltage value.

0.000

0.200

0.400

0.600

0.800

1.000

1.200

0 0.2 0.4 0.6 0.8 1 1.2

Re

l. P

DE

- m

on

och

rom

ato

r

Rel. PDE - pulse method

Relative PDE (470 nm), normalized to 1mm2, monochromator vs pulse method

W2-1B-2

W6-2A-1

W6-1B-1

50D-adv

W5-1B-1

W1-1A-2(epoxy)W1-3B-1

MPPC 1mm

MPPC, AP corr

diagonal

19

Summary and preliminary conclusions The relative PDE of KETEK SiPM prototype detectors produced for in 2012 for the LHCb SciFi project

were measured with a monochromator based set-up between 300 and 700 nm and with a zero-

pulse method at 470 nm. The agreement of the two methods is of the order 10%, where the zero-

pulse method appears to give in most cases higher values than the monochromator method. The

observed dependence on the overvoltage in the two methods is very similar.

The measurements show:

All KETEK arrays show similar spectral sensitivity curves as the 50D ADV detector which was

used as reference. The curves are narrower than the one of the measured HPK MPPC.

The peak values are within ±20% from the 50D ADV detector. The wavelength of maximum

PDE lies between 420 and 440 nm. The W1-1B-2 gives the highest PDE. The W1-3B-1 with

trenches the lowest.

Acknowledgement We would like to thank Thomas Schneider, Miranda van Stenis and Claude David from the PH/DT

Thin Film Lab for their precious technical support.

-25%

-20%

-15%

-10%

-5%

0%

5%

10%

0.00 1.00 2.00 3.00 4.00 5.00

rel.

PD

E d

iffe

ren

ce

OV (V)

Rel. PDE difference (470 nm) monochromator - pulse method

W2-1B-2

W6-2A-1

W6-1B-1

50D-adv

W5-1B-1

W1-1A-2(epoxy)W1-3B-1

MPPC 1mm

MPPC AP corr